Icom IC-701 All Band HF All Solid State Transceiver

Spured on by the success I have had fixing transceivers lately I decided to have another look at an Icom IC-701 I have had in the junk box for two years. This was the first "modern" rig I got when I decided to get back into radio. It was given to me with the warning it was not working. This is an interesting "Rig" in that it is one of the first to use a Digital Phase Locked Loop Synthesiser.

I soon found out why it was not working, someone had tried to repair a fault and had lost some of the parts they had removed. I pulled the bottom off and there was the bottom wafer of the Ledex switch swinging in the breeze. See the fourth photo. I searched the web and from what I found I put it in the too hard basket. This switch has two poles (wafers) to switch the various output filters in between the transmitter PA and the antenna socket. There is also another pole (wafer) that tells the control system the switch has switched in the correct filter for that band. The switch is driven round by what is called a Ledex Switch, a sort of stepping relay that rotates the switch shaft one position at a time.

Had another look at it recently and decided to see What I could do with it. I found some screws that fited the Ledex switch to hold the bottom wafer back on. Put the screws in and powered up the little beast for the first time. The digital dial read 29 MHz and when I rotated the switch the indicated bands changed. Great stuff. The electronics made no attempt to rotate the switch at first.

After a while it tried to shift the switch itself when I rotated it by hand and now when I rotate the band switch it clicks but does not seem to have the power to rotate the wafers. If I rotate it until it stops clicking it settles down on that band and it receives, quite well. Have been using it to listen to the action on 20M during the daylight hours.

The last photo above shows the bottom of the switch and I presume there is a bush missing and perhaps even a bottom support plate. Would anyone be able to take a photo of what it should look like and send to me ?.

Success at last: While recovering from a knee replacment and not have much else to do, I put the 701 on the bench again and worked out how to get the heatsink assembly off the back of the radio. I soon discovered why the switch did not rotate. The fellow who was trying to fix it had put the switch back together in such a way the index plate/spring that stopped the ratchet wheel going back to where it had come from after a step, was bent back on itself and the switch was jamed. It took quite a bit of force to roatate it, more than the electro magnet could provide. Put it back how it should be, put the radio back together and I was greeted with the satisfying sounds of the switch stepping its way round when I changed bands. Put in on air the next night on 80 Metres and the contacts reported it sounding very good. A problem developed the next night I used it, it sometimes does not lock on the correct frequency on the 80 Metre band and this fault got to a point where id did not work on 80M at all. This fault is described below.

I soon discovered it was off frequency only on 80 Metres! I asked around about this problem and Michael sent me back the following replay:

"I suggest you have a look at page 38 of the manual. You might need to touch up the VCO oscillator for 80 meters. That would be L4. If you've not done this before, please be careful adjusting the ferrite slug in the coil form. I was adjusting L6 for 40 meters and turned the ferrite slug too far. The slug hit the bottom of the coil and circuit board and I ended up breaking off the adjustment slot on top of the slug. After 35 years, those ferrite slugs are brittle!"

When I got the time and plucked up the courage again to fiddle with it I soon discovered Michael was right about the oscillator associated with L4, the frequency from coax two which should be 12.510 MHz when on 80M LSB was anything but most of the time.

All other frequencies comming from Coax 2 are spot on but the frequency when the radio is tuned to 3.5MHz LSB is 12.527 to about 12.535 instead of 12.510 and no amount of twiddling the slug would change the frequency. The next experiment is did was to see if changing from LSB to USB would produce the desired 3KHZ shift and it did not. I then changed bands a few times looking at the frequency shift LSB/USB and they were all like Icom intended, just as shown on page 38 of the manual. While switching from band to band several times looking at the frequencies, suddenly the 3.5MHz range gave me a frequency of 12.510 and the shift to USB gave me 12.513, but only for a few seconds now and off it went again.

What does the LSB/USB switch do that may be causing this ? All other bands are spot on the book frequencies.Just as well I have not pulled the PLL unit out yet, I may have attacked L4 and there may have been nothing wrong with it. I am begining to think the problem could be with the crystal or one of the varicap diodes in this oscillator. Any thoughts on this problem will be greatly appreciated.

Back to the shack equipment pages
Back to the Ham Radio pages
Back to the Domestic Radio pages
Back to opening page

Written by and Copyright, Phil. Storr © Last updated 9th Spetember 2013